1. Field of the Invention
The present invention relates to an atomic layer deposition apparatus, and more particularly, to an atomic layer deposition apparatus for uniformly depositing a thin atomic layer on multiple substrates.
2. Description of the Related Art
As the size of semiconductor memory devices decreases, the technology for growing a uniform thin film with respect to depression of a fine pattern has become the focus of much interest. Such a thin film growing technology includes sputtering, chemical vapor deposition (CVD) and atomic layer deposition (ALD) methods.
According to the sputtering method, argon (Ar) gas as an inert gas is injected into a vacuum chamber in which high voltage is applied to the target, to generate Ar ions in a plasma state. Then, Ar ions are sputtered onto the surface of a target such that atoms of the target, which become dislodged from the surface of the target, are deposited on a substrate. High purity thin film with a good adhesion to the substrates can be formed by sputtering method. However, in the deposition of a thin film having a depression with a step difference, it is difficult to ensure uniformity over the entire thin film. Thus, application of the sputtering method to a fine pattern is very limited.
In the CVD method, the most widely utilized deposition technique, a desired layer is deposited on a substrate to a desired thickness, using decomposition and reaction of gases. First, various gases are injected into a reaction chamber and chemical reactions between the gases are induced with high energy such as heat, light or plasma, to deposit a thin film having a predetermined thickness on a substrate. The deposition rate of CVD can be increased by controlling the reaction condition of heat, light or plasma supplied as a reaction energy, or the amount and ratio of gases. However, the reactions are so rapid that it is difficult to control the thermodynamic stability of atoms. The CVD deteriorates physical, chemical and electrical properties of the thin films, and the uniformity of a thin film having a fine depression cannot be ensured as in the sputtering method.
In addition, the ALD method is for depositing an atomic layer by alternately supplying a reaction gas and a purging gas. A thin film having a high aspect ratio, uniformity in a depression, and good electrical and physical properties, can be formed by the ALD method. Also, the films deposited by the ALD method have lower impurity density than those formed by other deposition methods and a thickness of less than 200 .ANG. is obtained with reproducibility.
For example, assuming that two reaction gases A and B are used, when only the reaction gas A flows into a reaction chamber, atoms of the reaction gas A are chemically adsorbed on a substrate (step 1). Then, the remaining reaction gas A is purged with an inert gas such as Ar or nitrogen (N.sub.2) (step 2). Then, the reaction gas B flows in, and a chemical reaction between the reaction gases A and B occurs only on the surface of the substrate on which the reaction gas A has been adsorbed, resulting in an atomic layer on the substrate (step 3).
Thus, the ALD method used for depositing a thin film on a substrate can ensure perfect step coverage regardless of the morphology of the substrate. Also, the remaining gas B and the by-products of the reaction between two gases A and B are purged (step 4), and the thickness of the thin film can be increased by repeating the four steps. In other words, the thickness of the thin film can be adjusted in atomic layer units according to the number of repetitions.
FIG. 1 is a schematic diagram of a conventional ALD apparatus.
As shown in FIG. 1, the ALD apparatus comprises a vacuum chamber 10 and a heater 11 for heating a substrate 13 placed in the vacuum chamber 10 to an appropriate temperature. The substrate 13 is seated on a substrate holder (not shown) placed on top of the heater 11, and heated evenly by the heater 11. Also, a showerhead 15 through which a predetermined reaction gas flows into the vacuum chamber 10, is installed facing the surface of the substrate 13.
In the ALD apparatus, a predetermined reaction gas flows into the vacuum chamber 10 heated to an appropriate temperature, through the showerhead 15, and the reaction gas which has flowed into the vacuum chamber 10, is deposited on a fine pattern of the substrate 13 as one atomic-layer film. After deposition, the remaining gas is purged by a purging gas which is flowed following the reaction gas, to be discharged to the outside.
FIG. 2 is a schematic diagram of another conventional ALD apparatus.
As shown in FIG. 2, the ALD apparatus comprises a vacuum chamber 20 and a reactor 20a installed in the vacuum chamber 20. The reactor 20a is opened by the upper half thereof, and heaters 21 are installed above and below the reactor 20a. A single substrate 23 is seated in the reactor 20a and is heated evenly by the heaters 21.
In the ALD apparatus, a predetermined reaction gas flows in the reactor 20a through an inlet 25 and is then deposited as a thin film on the substrate 23. The remaining reaction gas is purged by a purging gas which is flowed following the reaction gas, to be discharged to the outside through an outlet 26.
However, while the conventional ALD apparatus can provide a thin film having a high aspect ratio, in addition to having a good uniformity over a depression, it can not be commercialized due to its low deposition rate and a separated deposition for each substrate.
The deposition rate in the conventional ALD apparatus can be increased by increasing the activation energy of gases. However, increasing the activation energy of gases causes CVD on the substrate, instead of forming an atomic layer, thereby negating the merits of the ALD method, such as good film properties or uniformity of a thin film over a depression.